I had this crazy idea since I'd like to get a Kel-tec Sub 2000. Is it possible to add a stock spacer to gain the overall length, and configure the Sub-2000 to be featureless?

The main issue is the pistol grip, an idea is to hacksaw the whole thing off. I would leave the area next to the mag catch. The rifle would not have the capacity to have a pistol like grip and this would be obvious without the magazine. With the magazine I guess you would have to pinch it and shoot it like that.